Selecting Premium Efficient Motors

March 1, 2004
Electric motor–driven industrial equipment consumes 64 percent of the electricity used in U.S. industry according to a U.S. Department of Energy (DOE) report published in 1998.

This energy consumption may be reduced by as much as 18 percent if companies adopted “proven efficiency technologies and practices.” Much of the savings can come from installing new premium efficient motors in place of older, less efficient designs.

According to DOE data, only about 10 percent of the motors in the United States comply with the minimum efficiency levels mandated by the Energy Policy Act (EPAct) that took effect in October 1997. Motors with efficiency levels significantly higher than the EPAct mandated minimums are available for even more energy savings. These motors are termed “premium efficient” motors. Besides decreasing energy costs, these premium efficient motors offer many additional benefits.

Premium efficient motors are more efficient at full load because of reductions in their internal losses. These motors also offer even greater efficiency when they are used to operate machinery at less than full load.

Almost all of the additional benefits relate to the lower temperature rise of premium efficiency motors, when compared to earlier, less efficient motors. Cooler running motors increase the life of the insulation systems and last longer. For every 10°C cooler the motor operates, the insulation system life doubles. Bearing and lubrication life is also increased.

Because the motors run cooler and the electrical design uses more active material, they have more tolerance to both low and high voltages. Low voltages are likely to occur as utilities try to avoid blackouts by taking voltage reductions (brownouts).

Premium efficient motors are generally rated for adjustable speed use with an inverter, a system that can provide significant energy savings. On a pump or fan application, use of an inverter to adjust the motor speed may save more than 50 percent of the energy used for the same motor operated at full speed when the flow is controlled by a valve or damper. These motors are termed “inverter-ready” and may be provided in either TEFC (totally-enclosed fan-cooled) or ODP (open drip-proof) enclosures.

Because internal motor losses are reduced, a smaller fan is needed for cooling. This smaller fan provides quieter operation than a standard motor.

Many premium efficient motors designs meet guidelines from the Consortium for Energy Efficiency (CEE), a national, non-profit public benefits corporation that promotes the manufacture and purchase of energy-efficient products and services. Efficiency guidelines from CEE are benchmarks used by many utilities when establishing their motor rebate programs.

Despite all of these overwhelming advantages, there are a few considerations to make when applying premium efficient motors.

To lower motor losses, the motor slip is reduced. This means that a premium efficient motor may run 1790 revolutions per minute (RPM) full load when an older motor may only run 1760 RPM. On a constant torque application such as a conveyor, this is generally no problem. But on a variable torque application, such as a centrifugal pump or fan, the load increases by the cube of the speed increase. This means that a 1 percent increase in speed equals a 3 percent increase in load, drawing more current. The fan and pump curves should be consulted when replacing motors on variable torque applications.

Many premium motors produce less starting torque than older motors. Although they are required to produce minimum starting torque defined by the National Electrical Manufacturers Association (NEMA), the older motors may produce much more than those minimum values. This means that a new motor may not start a load requiring high starting torque.

The energy savings for premium efficient motors are definitely worth the effort to do an analysis for their replacement. Any motor that operates continuously is a viable candidate. The user should not focus strictly on high horsepower motors, as they may already be the most efficient in the plant. Replacing many smaller motors—100 HP and below—can potentially offer the best payback results. As energy costs continue to rise, the savings will increase every year.

John Malinowski, [email protected], is motor product manager for Baldor Electric Co.